A secondary electron detector unit known from the Polish patent application No. P 329339, entitled “High pressure scanning electron microscope”, is composed of a secondary electron scintillator detector placed behind a microporous plate that is built into a vacuum wall separating an intermediate chamber and an electron optical column part. Here, the detector scintillator and the microporous plate are located coaxially with the electron optical column, along which the electron beam travels. A more functional solution for a secondary electron detector unit for a scanning electron microscope is known from the paper: W. Slówko entitled: “Electron signal acquisition in HPSEM”, Vacuum 2003, vol. 70, p. 157. In this solution a scintillator is placed aside from the electron optical axis. In both solutions, a stream of secondary electrons is guided into the intermediate chamber through a hole in a lower throttling aperture and impinges symmetrically the input surface of the microporous plate, which is placed at the electron optical axis. The electrons passing across the microporous plate are multiplied and, at the output side, they are attracted by the scintillator biased with a high voltage and finally detected. The active input surface of the microporous plate is ring-shaped and coaxial with the electron beam. In the central part, it is limited by the hole made on the axis of the microporous plate to let through the electron beam while its maximum diameter is limited by the opening in the frame plate, which also fixes the microporous plate in the frame body of the detector.